题目内容：

If he ___ to the teacher attentively,he ___ the answer to the problem now.
A.had listened,would have known B.listened,would know
C.listened,would have known D.had listened,would know

优质解答

If he ___ to the teacher attentively,he ___ the answer to the problem now
D.had listened,would know
这道题目是考查虚拟语气中的交叉虚拟问题.
即,当虚拟条件句中,主句和从句的时间是不一致的时候,应分别考虑不同时间下的虚拟状况.
按照本句的实际情况,listen发生的时间应在过去,那么对于过去时间的虚拟采用过去完成时,had listened ;而know the answer 发生在现在,那么对现在时间的虚拟采用would +动词原形,也就是would know